This invention relates to the preparation and use of alkyl-substituted tetrahydroanthraquinones, and more particularly to 1-alkyl-and 1,3-dialkyl-1,2,3,4-tetrahydroanthraquinones.
The use of alkyl-substituted anthraquinones and their tetrahydro derivatives in cyclic processes for the production of hydrogen peroxide is well known. Ferri Canadian Patent 614,296, issued Feb. 7, 1961, and the following U.S. patents (whose issue dates are shown in parentheses) are representative of the many issued patents relating to anthraquinone processes for producing hydrogen peroxide: Reidl et al. U.S. Pat. No. 2,158,525 (5/16/39) and U.S. Pat. No. 2,215,883 (9/24/40); Dawsey et al. U.S. Pat. No. 2,537,655 (1/9/51), Sprauer U.S. Pat. No. 2,657,980 (11/3/53), Harris et al. U.S. Pat. No. 2,668,753 (2/9/54) and Hinegardner U.S. Pat. No. 2,689,169 (9/14/54); and Darbee et al. U.S. Pat. No. 3,062,622 (11/6/62), Hiratsuka et al. U.S. Pat. No. 3,038,786 (6/12/62), Dawsey U.S. Pat. No. 3,041,143 (6/26/62), Kabisch U.S. Pat. No. 3,328,128 (6/27/67), Kabisch et al. U.S. Pat. No. 3,488,150 (1/6/70) and Logan et al. U.S. Pat. No. 3,493,343 (2/3/70). In some cases, the presence of a limited amount of the tetrahydro form is regarded as advantageous since a higher active quinone concentration in the working solution can be achieved. However, two disadvantages of the tetrahydro form are also recognized.
1. It is less soluble than the parent anthraquinone and will precipitate from solution more readily, and PA1 2. It is oxidized more slowly.
It is important in commercial operations of such anthraquinone processes that the working solution employed have a high hydrogen peroxide synthesis capacity per cycle and that the hydrogen peroxide solution obtained in the extraction step be relatively concentrated. The synthesis capacity per cycle is determined by the solubility of the working intermediate employed, particularly in its anthrahydroquinone form, and of course also by the particular solvent or solvent mixture used, which factors also determine the maximum concentration of hydrogen peroxide obtainable in the extract product solution.
U.S. Pat. No. 3,778,452 issued to Josey et al. on Dec. 11, 1973, the disclosure of which is incorporated herein by reference, discloses a 1,4,4a,9a-tetrahydro-1-alkenyl-9,10-anthraquinone of structural formula ##STR1## in which each of R.sub.1 and R.sub.4 is an alkyl group, or either is an alkyl group and the other is hydrogen; and each of R.sub.2 and R.sub.3 is an alkyl group or hydrogen, provided that R.sub.2 is always hydrogen when each of R.sub.1 and R.sub.3 is hydrogen; said alkyl group or groups being 1 to 8 carbon straight chain hydrocarbon alkyl groups.
Alkenyl anthraquinones can be converted to the corresponding tetrahydroalkylanthraquinones, which have excellent utility as working intermediates in the cyclic process for the production of hydrogen peroxide. This conversion is accomplished by aromatizing the 1-alkenyl-substituted ring with oxygen (air) in the presence of a strong base, followed by hydrogenation according to the process taught by the copending application of Kirchner and Vaughan, Ser. No. 285,124, filed Aug. 31, 1972, the disclosure of which is incorporated herein by reference. According to Kirchner and Vaughan, the alkenyl substituent is converted to the corresponding alkyl substituent ##STR2## by hydrogenation in the presence of a platinum catalyst on a charcoal support. Hydrogenation in the presence of Raney nickel catalyst converts this to 5,6,7,8-tetrahydro-1-alkylanthraquinones of the formula ##STR3##
Although a wide variety of tetrahydroanthraquinones has been disclosed in the literature, all are characterized by an unsubstituted tetrahydro ring.